This invention relates to a controller of a digital control system containing a program transmitter and a method for controlling the same.
In recent years, when automatically process controlling the temperature in a furnace utilized in heat treatment in a chemical factory, a program transmitter is used which outputs desired set point variables corresponding to respective times in accordance with a prestored program pattern, and the set point variables are processed by a digital arithmetic operation together with process variables, thus performing controls.
The process acting as the controlled system is generally a system having a high order log and a time delayed response resulting in useless time in the control. For this reason, in order to effect the control at a sufficiently high response speed while preventing departure from a stable condition, a control based only on a proportional action is not sufficient. Accordingly, a proportional plus integral plus derivative ("PID") action in which integral action and a derivative action are added to the proportional action has been used. In this case, PID parameters, i.e. proportional band (PB) or proportional gain, reset time (TI), and derivative time (TD) or rate time, are determined in accordance with respective processes, and the controls are made in accordance with the parameters.
However, in a process, the control actions of the process vary as time elapses at respective processing steps (hereinafter termed time segments) during the process in accordance with the magnitude of the set point variable, for example the level of the temperature or the characteristics of the treated objects. In the case of controlling furnace temperature, the control actions are not the same where the temperature to be controlled is relatively low or high.
With the prior art controller, only one set of PID parameters was set for one process. Although a prior art controller performed a satisfactory control action based on a PID suitable for a given segment, that control action and PID was not always satisfactory for other segments. As a consequence, it has been difficult to ensure adequate and accurate controls throughout the process.